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My husband and I moved into our new home in November 2000 and immediately began buying and planting citrus trees, including several varieties of oranges, grapefruit, Meyer lemon, key lime and Persian lime. A few years later they were all gone due to the eradication efforts put into effect to eliminate citrus canker. This method of destroying all the trees within 50 feet of an infected tree was successful several times since 1910 which is when the disease was introduced into Florida. In 1933 canker was considered eradicated at a cost of $6.5 million.
Then in 1995, canker was discovered in Miami-Dade County. Despite a 10-year monumental effort to eradicate the disease from Florida, a combination of challenges and a series of hurricanes in 2004 and 2005 spread the disease to the point where eradication was no longer possible. At present eradication has been deemed unfeasible due to a number of factors, primarily the cost. There is still a quarantine regarding shipping or taking citrus across state lines. The story is that someone brought an infected tree from Miami into Cape Coral in the early 2000s and since the disease is airborne, it spread like wildfire.
So what exactly is citrus canker? It is caused by the bacterium Xanthomonas citri and likely originated in southern Asia. The disease can now be found essentially worldwide. While not harmful to humans, canker significantly affects the vitality of citrus trees, causing leaves and fruit to drop prematurely. A fruit infected with canker is safe to eat but has reduced marketability as fresh fruit. The bacterium is extremely contagious and is spread rapidly when the summer rains and high temperatures are common. Besides rainwater, irrigation water will spread the bacteria from plant to plant. Wind events such as hurricanes, birds, animals, people moving contaminated equipment, tools, tree clippings, untreated infected fruit and infected plants will also introduce the bacteria to a new plant.
The Asian leaf miners (Phyllocnistis citrella), evident by the tunnels produced in the middle of the leaf, do not act as vectors, but spread the disease through damage they cause in the foliage when they feed. Controlling leaf miners is one way of controlling the disease. The bacteria enter the leaves through microscopic holes called stomata or through wounds caused by weather damage or insects such as the leaf miner. Young leaves are most susceptible. Citrus species such as grapefruit, lime and lemon are most susceptible to this disease.
How does one know whether their citrus tree has canker? Symptoms of citrus canker are raised lesions that can be found on both sides of the leaf, and on the stems and fruit. Characteristic lesions are raised, have a crater-like appearance surrounded by concentric circles. The lesions may have a water-soaked margin and a corky texture. As the disease progresses, the lesions may be surrounded by a yellow halo.
To your dismay, one of your citrus trees has canker. What do you do? The treatment recommended is to spray the tree with preventive copper based bactericides. Copper sulfate is a common product in any nursery or big box store. Follow the recipe (instructions) on the side of the bottle and spray when there are leaf flushes, which typically requires spraying about every three to four weeks from spring through fall. Spraying is used in conjunction with cultural practices such as pruning of diseased summer and fall shoots. Put infected plant parts in the garbage and not in the horticulture pick up.
To control leaf miners, use products containing imidacloprid, such as Bayer Advanced Fruit, Citrus, and Vegetable Insect Control. Use it as a drench or foliar spray. This generally is a once a year application.
Foliar sprays of natural materials such as Azadirachin (Safer BioNEEM) or spinosad (Green Light Insect Spray) have some efficacy but will need to be repeated.
There are no fully canker resistant varieties commercially available, but not all citrus varieties are equally susceptible. Tangerines are tolerant to the disease and Valencia oranges, tangelos and other mandarin hybrids only become infected under conditions that are optimal for disease spread like a hurricane.
Take care and enjoy the wonderful taste of citrus from your own yard.
Kathy Woodlock is Past President of the Garden Club of Cape Coral, a Master Gardener/Citrus Advisor, flower show judge and landscape consultant.